Continuously variable transmissions (CVT) of the variable ratio belt type (CVU) include a planetary gearset which provides reversing operation. The planetary gearset is positioned upstream, in a power flow direction, of the friction belt. This reduces the torque capacity requirement of the planetary gearset which reduces the weight and size thereof.
There is, however, a disadvantage to placing the reversing operation upstream of the CVU. A negative one (-1) ratio cannot be attained with a simple planetary gearset. If the planetary ratio during reversing operation is greater than unity, the belt tension in the CVU can exceed maximum design criteria or the input torque must be reduced (i.e., the engine output and/or torque converter ratio is limited). Thus, a compound (double pinion) planetary gearset is used to establish the reverse operation. This increases the complexity and cost of the planetary gearset.
In most current automatic shifting transmissions, the reverse torque, by design, is approximately 70% of the low forward ratio, while in CVTs, the reverse torque is approximately 80% of the low forward ratio. During reverse operation, the CVU is usually maintained at a fixed underdrive ratio in most CVT mechanisms.